1. Field of the Invention
This invention relates to a process for the elimination of sewage odor by adding a solution of sulfur dioxide in sodium hydroxide, or a solution of sulfite, bisulfite, metabisulfite, sulfurous acid, or sulfur dioxide to sewage, sewage conduit, and sewage systems in the presence of metal catalysts. The invention also prevents further formation of odoriferous compounds in sewage, and sulfide-induced corrosion.
The invention is particularly addressed to the problem of recovering sewage from its septicity. This invention is also directed to the initiation and promotion of biosynthesis by uptake of oxygen, and purification of sewage and wastewater by increasing the efficiency of the biological process.
2. Description of the Prior Art
The characteristic of sewage is its highly disagreeable odor. Many organic and inorganic compounds contribute to the formation of sewage odor. Mostly inorganic and organic sulfur compounds are held responsible for the odor. The principal among the sulfur compounds is hydrogen sulfide. It is also easy to detect. When almost no hydrogen sulfide formation takes place in sewage, it is considered odorless. Therefore, hydrogen sulfide is taken as a representative of odor forming compounds. Hydrogen sulfide in sewage creates a public nuisance because of its highly disagreeable odor at levels as low as 0.01 parts per million (PPM) in the atmosphere. Breathing by humans of air containing as low as 10 PPM of hydrogen sulfide for a period of time can prove fatal. Fatal accumulations are likely to occur in covered or domed clarifiers or settling takes in treatment plants, or in manholes or wet wells in sewerage systems.
Besides odor, hydrogen sulfide formation results in heavy corrosion of metal and concrete sewer lines, concrete and metal structures, and metal equipment and machinery. The corrosion is greatly reduced when the formation of hydrogen sulfide can be reduced in sewage. So far as it is known, the presence of hydrogen sulfide is also detrimental to the growth of the microorganisms.
Hydrogen sulfide is not easily oxidized. It can exist in aqueous solutions supersaturated with oxygen. The occurrence of hydrogen sulfide in the Baltic Sea, the North Sea, and the Atlantic Ocean supersaturated with oxygen has been reported (Leck, C. and Bagender, L. E., Anal. Chem., 1988, 1680-1683). Although hydrogen sulfide is almost always formed in sewage, sewage conduit, and sewage systems, the rate of formation is greatly enhanced by summer temperature and prolonged anaerobic conditions.
The conditions under which hydrogen sulfide is formed in sewage are called septic. In fact, the microorganisms present in sewage turn black and seem lifeless or in serious metabolic disorder in septic sewage. Until this condition is reversed, sewage can not be efficiently purified by biological process.
Although many treatments to control formation and regeneration of sewage hydrogen sulfide have been tried, none has been uniformly satisfactory or successful in both gravity and force main systems. Among those treatments used have been aeration, chlorination, ozonation, lime, sodium nitrate, activated carbon filtration, odor masking, hydrogen peroxide oxidation, and iron salts.
Ferric chloride is widely used to control odor as well for separation of phosphate in many treatment plants with moderate success. Many treatment plants in Europe and some in America have started to treat sewage with pure oxygen. By treating with pure oxygen, the problem of odor, corrosion, and septicity have largely been overcome. The use of pure oxygen for controlling odor and as a source of molecular oxygen for the activated sludge reactors is the best known process for purifying sewage. But this is not a very satisfactory process. One plant in Chicago which injects in the force pure oxygen to control hydrogen sulfide has to add hydrogen peroxide in summer months in addition to pure oxygen to control hydrogen sulfide.
Pure oxygen is also expensive. It is not a cost-effective process for small plants. Many treatment plants in America are trying to explore some other alternative methods.
To evaluate the possibility of inventing an alternative method for controlling sewage odor, a literature search was conducted. Several patents were discovered which used sulfur compounds for purification of sewage and wastewater in the presence of iron and other metals preferably at a low pH. All the patents discuss methods for purification of water by chemical oxidation. No attempts have been made to purify sewage or wastewater by improving biological process by taking advantage of oxidative power that is generated in the presence of a metal catalyst and oxygen. None of these methods has been commercially accepted.
One example is U.S. Pat. No. 653,741, issued to Jewell in 1900, which involves the use of sulfurous acid solution and scrap iron to produce a reactant solution that is mixed with water to be treated in a liquid process. U.S. Pat. No. 2,171,203 to Urbain et al. teaches purification of organically polluted water such as sewage by generating Fe.sub.2 (SO.sub.4).sub.3 in situ by passing sulfur dioxide in sewage in the presence of iron. Additionally, U.S. Pat. No. 3,522,173 to Lindman et al. teaches the treatment of wastewater by the use of sulfur dioxide as an acidifying agent and iron as a flocculating agent to purify the wastewater. However, the Lindman et al. process requires mixing the sulfur dioxide gas stream with large amounts of oxygen or oxygen-containing gases for the method to be effective. U.S. Pat. No. 3,948,774 issued Apr. 6, 1976 involves purification of water by treating wastewater with sulfur dioxide and iron between pH 2.4 and 2.6. Further, the use of sulfur dioxide for purification of wastewater is described in U.S. Pat. No. 4,123,355. In the process, sufficient sulfur dioxide is used to reduce the pH of the wastewater, generally to the order of pH 2 to 3.
Two patents to MacLachlan, U.S. Pat. Nos. 1,511,418 and 1,543,939, each discuss the treatment of sewage sludge with sulfur dioxide gas. However, the patents make no mention of the process as being applicable to water treatment, and neither patent discusses the use of the method for water purification purposes.
Sulfur dioxide is also used as disinfectant. U.S. Pat. Nos. 4,304,673 and 4,340,489 discuss processes by which wastewater is continuously disinfected by combining the wastewater with sufficient sulfur dioxide so that the wastewater has a selected free sulfur dioxide content of at least 5 mg/liter.
Cyanide can be almost completely oxidized by taking advantage of the oxidative power generated by sulfur dioxide oxidation. U.S. Pat. Nos. 4,537,686; 4,615,873, and 4,622,149 claim that the cyanide content of industrial wastewater containing the same is removed by treatment with sulfur dioxide or an alkali or alkaline earth metal sulfite or bisulfite in the presence of excess oxygen and a metal catalyst, preferably copper.
U.S. Pat. No. 2,877,177 issued Mar. 10, 1959 discusses a process of treating sulfur dioxide waste liquors containing alkali metal sulfides which are highly colored and odoriferous with sulfurous acid and thereby converting them into colorless and odorless liquids.
In all the patents it appears that the oxidation of sulfur dioxide generates oxidizing power. Thus water has been purified by oxidizing sewage and wastewater chemically. But no attempts have been made to purify sewage and wastewater by biological process after modifying and improving the condition of sewage with sulfite or sulfur dioxide addition.
Recently, it has been claimed by Kotronarou et al. in an article in Research Journal, Water Pollution Control Federation, Nov. 1991, that peroxymonosulfate is a more rapid and efficient oxidant of hydrogen sulfide and as a viable alternative to hydrogen peroxide for the control of sulfide-induced corrosion in concrete sewers.
In order to understand the condition of sewage when it is confined in sewage conduit for a long time in the absence of oxygen and at high temperatures, a study was made in the laboratory with sulfites and activated sludge. During the course of the study, sulfite, bisulfite, metabisulfite, as solution or sulfur dioxide in sodium hydroxide, and sulfurous acid were added together with metal salts to odoriferous black septic activated sludge. The black color of the activated sludge gradually turned gray and odor was greatly reduced. The gray activated sludge turned black again when it was allowed to sit in the laboratory in the absence of oxygen and gradually it became an increasing source of odor formation. However, on addition of sulfite and iron salts, the black sludge again regained their gray healthy look and odor also was reduced. Thus it appeared that sulfite could prevent activated sludge from being septic and was essential for the metabolism of the activated sludge.
It is also believed that sulfur in the form of sulfite enters in biological reaction. Thus it appeared that sulfite has a great potential in controlling sewage odor, recovering sewage from septicity increasing oxygen uptake in sewage, and promoting microbial metabolism, but sulfur dioxide or sulfite had never been used as such. The problem of controlling sewage odor and purification of sewage in a cost-effective way by improving biological process basically has not been solved.
It is, therefore, an object of the present invention to provide a process for the elimination of odor in sewage, sewage conduit, and sewage systems by aerating and sulfur dioxide-sodium hydroxide treatment in the presence of metal catalysts Instead of sulfur dioxide-sodium hydroxide, sulfite, bisulfite, meta or pyrosulfite, sulfur dioxide and a solution of sulfur dioxide in water can be used. Metal salts such as iron, copper, nickel, manganese, chromium, titanium, vanadium, and other metals and compounds such as zinc, platimum peroxydisulfate and the like which promote oxidation of sulfur dioxide or sulfite may be used.
It is a particular object of the present invention to provide a process for the recovery of sewage from septicity, increase uptake rate of oxygen in sewage, and initiate and promote microbial metabolism in sewage. It is also an object of the present invention to utilize oxygen injected in sewage and activated sludge reactors as fully as possible by increasing oxygen absorbing capacity of sewage and of the activated sludge and thereby reducing the cost of operation.
It is also an object of the invention to provide a process by which sulfide-induced corrosion of sewage treatment plants can be overcome.